Energy harvesting device using SSHI techniques

ABSTRACT

An energy harvesting device harvests energy from an energy source, and includes an inductor and a control switch coupled in series, and a control module. The series connection of the inductor and the control switch is adapted to be coupled to the energy source in parallel or in series. The control module is coupled to the control switch, and controls operation of the control switch between an ON state and an OFF state such that the control switch starts to operate in the ON state for a predetermined time period from a transition time point during each predetermined cycle starting from a start time point, and such that a time difference between the transition time point and the start time point is variable.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Taiwanese Application No. 103109103,filed on Mar. 13, 2014, the contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an energy harvesting device, and moreparticularly to an energy harvesting device using SSHI (synchronizedswitch harvesting on an inductor) techniques.

2. Description of the Related Art

Referring to FIGS. 1 and 2, two conventional energy harvesting devices11, 11′ are disclosed in “Revisit of series-SSHI with comparisons toother interfacing circuits in piezoelectric energy harvesting” by I. C.Lien et al., Smart Materials and Structures, vol. 19, no. 12, pp.125009, 2010.

The first conventional energy harvesting device 11 is coupled between apiezoelectric vibrating device 12 and a load 13. The piezoelectricvibrating device 12 vibrates at an operating frequency in response to aforce applied thereto, and converts the vibration into an AC voltage.The first conventional energy harvesting device 11 converts the ACvoltage from the piezoelectric vibrating device 12 into a DC voltage,and outputs the DC voltage to the load 13. The first conventional energyharvesting device 11 uses parallel-SSHI techniques, where a seriesconnection of a switch 111 and an inductor 112 is coupled to thepiezoelectric vibrating device 12 in parallel, and where the switch 111operates in an ON state for a predetermined time period each time avibration displacement of the piezoelectric vibrating device 12 reachesits extremum (i.e., a phase difference between a center time point ofthe predetermined time period and a respective time point at which thevibration displacement reaches its extremum is zero). As a result,energy harvested by the first conventional energy harvesting device 11from the piezoelectric vibrating device 12 reaches its maximum when theoperating frequency of the piezoelectric vibrating device 12 is equal toa short circuit resonant frequency of the same. However, the harvestedenergy decreases rapidly with deviation of the operating frequency fromthe short circuit resonant frequency.

The second conventional energy harvesting device 11′ differs from thefirst conventional energy harvesting device 11 in that the secondconventional energy harvesting device 11′ uses series-SSHI techniques,where the series connection of the switch 111 and the inductor 112 iscoupled to the piezoelectric vibrating device 12 in series, and wherethe switch 111 operates in the ON state for the predetermined timeperiod each time the vibration displacement of the piezoelectricvibrating device 12 reaches its extremum (i.e., the phase differencebetween the center time point of the predetermined time period and therespective time point at which the vibration displacement reaches itsextremum is zero). As a result, energy harvested by the secondconventional energy harvesting device 11′ from the piezoelectricvibrating device 12 reaches its maximum when the operating frequency ofthe piezoelectric vibrating device 12 is equal to an open circuitresonant frequency of the same. However, the harvested energy decreasesrapidly with deviation of the operating frequency from the open circuitresonant frequency.

SUMMARY OF THE INVENTION

Therefore, an object of this invention is to provide an energyharvesting device that can overcome the aforesaid drawbacks associatedwith the prior art.

According to one aspect of this invention, there is provided an energyharvesting device for harvesting energy from an energy source. Theenergy harvesting device comprises an inductor and a control switchcoupled in series, and a control module. The series connection of theinductor and the control switch is adapted to be coupled to the energysource in parallel or in series. The control module is coupled to thecontrol switch, and controls operation of the control switch between anON state and an OFF state such that the control switch starts to operatein the ON state for a predetermined time period from a transition timepoint during each predetermined cycle starting from a start time point,and such that a time difference between the transition time point andthe start time point is variable.

According to another aspect of this invention, there is provided anenergy harvesting device for harvesting energy from an energy sourcethat outputs an alternating current (AC) voltage. The energy harvestingdevice comprises an AC-to-DC (direct current) converting module, aninductor, a control switch, a filtering capacitor and a control module.The AC-to-DC converting module has two input terminals adapted to becoupled to the energy source for receiving the AC voltage therefrom, andtwo output terminals. The AC-to-DC converting module converts the ACvoltage into a DC voltage, and outputs the DC voltage at the outputterminals. The inductor and the control switch are coupled in seriesbetween the input terminals of the AC-to-DC converting module. Thefiltering capacitor is coupled between the output terminals of theAC-to-DC converting module. The control module is coupled to the controlswitch, and controls operation of the control switch between an ON stateand an OFF state such that the control switch starts to operate in theON state for a predetermined time period from a transition time pointduring each predetermined cycle starting from a start time point, andsuch that a time difference between the transition time point and thestart time point is variable.

According to yet another aspect of this invention, there is provided anenergy harvesting device for harvesting energy from an energy sourcethat outputs an AC voltage. The energy harvesting device comprises anAC-to-DC converting module, an inductor, a control switch, a filteringcapacitor and a control module. The AC-to-DC converting module has twoinput terminals adapted to be coupled to the energy source for receivingthe AC voltage therefrom, and two output terminals. The AC-to-DCconverting module converts the AC voltage into a DC voltage, and outputsthe DC voltage at the output terminals. The inductor, the control switchand the filtering capacitor are coupled in series between the outputterminals of the AC-to-DC converting module. The control module iscoupled to the control switch, and controls operation of the controlswitch between an ON state and an OFF state such that the control switchstarts to operate in the ON state for a predetermined time period from atransition time point during each predetermined cycle starting from astart time point, and such that a time difference between the transitiontime point and the start time point is variable.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of this invention will become apparent inthe following detailed description of the preferred embodiments of thisinvention with reference to the accompanying drawings, of which:

FIG. 1 is a schematic circuit diagram illustrating a first conventionalenergy harvesting device;

FIG. 2 is a schematic circuit diagram illustrating a second conventionalenergy harvesting device;

FIG. 3 is a schematic circuit diagram illustrating the first and secondpreferred embodiments of an energy harvesting device according to thisinvention;

FIG. 4 is an exemplary timing diagram illustrating a source current ofan energy source and a state of a first control switch of the energyharvesting device when an operating frequency of the energy source issmaller than a short circuit resonant frequency of the same;

FIG. 5 is an exemplary timing diagram illustrating the source current ofthe energy source and the state of the first control switch of theenergy harvesting device when the operating frequency of the energysource is equal to the short circuit resonant frequency of the same;

FIG. 6 is an exemplary timing diagram illustrating the source current ofthe energy source and the state of the first control switch of theenergy harvesting device when the operating frequency of the energysource is greater than the short circuit resonant frequency of the same;

FIG. 7 is a diagram illustrating a relationship between the operatingfrequency and an output power of the energy source in variousconditions;

FIG. 8 is a schematic circuit diagram illustrating the third preferredembodiment of an energy harvesting device according to this invention;

FIG. 9 is a schematic circuit diagram illustrating the fourth and fifthpreferred embodiments of an energy harvesting device according to thisinvention;

FIG. 10 is an exemplary timing diagram illustrating a source current ofan energy source and states of first and second control switches of theenergy harvesting device t when an operating frequency of the energysource is smaller than an open circuit resonant frequency of the same;

FIG. 11 is an exemplary timing diagram illustrating the source currentof the energy source and the states of the first and second controlswitches of the energy harvesting device when the operating frequency ofthe energy source is equal to the open circuit resonant frequency of thesame;

FIG. 12 is an exemplary timing diagram illustrating the source currentof the energy source and the states of the first and second controlswitches of the energy harvesting device when the operating frequency ofthe energy source is greater than the open circuit resonant frequency ofthe same;

FIG. 13 is a schematic circuit diagram illustrating a first modificationof the fourth preferred embodiment;

FIG. 14 is a schematic circuit diagram illustrating a secondmodification of the fourth preferred embodiment; and

FIG. 15 is a schematic circuit diagram illustrating the sixth preferredembodiment of an energy harvesting device according to this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before this invention is described in greater detail with reference tothe accompanying preferred embodiments, it should be noted herein thatlike elements are denoted by the same reference numerals throughout thedisclosure.

Before describing this invention in detail, it should be noted hereinthat throughout this disclosure, when two elements are described asbeing “coupled in series,” “connected in series” or the like, it ismerely intended to portray a serial connection between the two elementswithout necessarily implying that the currents flowing through the twoelements are identical to each other and without limiting whether or notan additional element is coupled to a common node between the twoelements. Essentially, “a series connection of elements,” “a seriescoupling of elements” or the like as used throughout this disclosureshould be interpreted as being such when looking at those elementsalone.

Referring to FIG. 3, the first preferred embodiment of an energyharvesting device 2 according to this invention harvests energy from anenergy source 3 that equivalently includes at least an alternatingcurrent (AC) supply 31 and an impedance circuit 32. The AC supply 31supplies a source voltage and a source current I_(S)(t) to the impedancecircuit 32, and the impedance circuit 32 outputs an AC voltage and an ACcurrent. In this embodiment, the energy source 3 is a piezoelectricvibrating device that vibrates at an operating frequency in response toa force applied thereto and that converts the vibration intoelectricity. Therefore, the energy source 3 further includes a clampcapacitor 33, the source voltage supplied by the AC supply 31 ispositively proportional to the force, and the impedance circuit 32includes a source resistor 321, a source inductor 322 and a sourcecapacitor 323 coupled in series between the AC supply 31 and the clampcapacitor 33, and outputs the AC voltage and the AC current across theclamp capacitor 33. However, in other embodiments, the energy source 3may be a device that receives and converts electromagnetic radiationinto electricity, or a device that receives electricity throughinductive coupling.

The energy harvesting device 2 uses parallel-SSHI (synchronized switchharvesting on an inductor) techniques, and includes an AC-to-DC (directcurrent) converting module 21, an inductor 22, first and second controlswitches 23, 24, a filtering capacitor 25, an impedance emulation module26 and a control module 27.

The AC-to-DC converting module 21 includes first and second inputterminals 211, 212 adapted to be coupled to the energy source 3 forreceiving the AC voltage therefrom, first and second output terminals213, 214 for outputting a DC voltage, and first to fourth convertingswitches 215-218. The first converting switch 215 is coupled between thefirst input terminal 211 and the first output terminal 213. The secondconverting switch 216 is coupled between the second input terminal 212and the first output terminal 213. The third converting switch 217 iscoupled between the first input terminal 211 and the second outputterminal 214. The fourth converting switch 218 is coupled between thesecond input terminal 212 and the second output terminal 214.

The inductor 22 and the first control switch 23 are coupled in seriesbetween the first and second input terminals 211, 212 of the AC-to-DCconverting module 21 (i.e., the series connection of the inductor 22 andthe first control switch 23 is coupled to the energy source 3 inparallel). The impedance emulation module 26 is coupled to the filteringcapacitor 25 in parallel, and has a variable impedance. The secondcontrol switch 24 and the parallel connection of the filtering capacitor25 and the impedance emulation module 26 are coupled in series betweenthe first and second output terminals 213, 214 of the AC-to-DCconverting module 21. The impedance emulation module 26 may usetechniques disclosed in “Resistor Emulation Approach to Low-Power RFEnergy Harvesting” by T. Paing et al., IEEE Transactions on PowerElectronics, vol. 23, no. 3, pp. 1494, 2008, and details thereof areomitted herein for the sake of brevity.

The control module 27 is coupled to the AC-to-DC converting module 21,the first and second control switches 23, 24 and the impedance emulationmodule 26, is adapted to be coupled to the energy source 3 for detectingthe AC voltage and the AC current therefrom, and obtains an output powerof the energy source 3 based on the detected AC voltage and the detectedAC current.

The control module 27 controls, based on the detected AC voltage,operation of each of the first to fourth converting switches 215-218 ofthe AC-to-DC converting module 21 between an ON state and an OFF statesuch that the first and fourth converting switches 215, 218 operate inthe ON state and the second and third converting switches 216, 217operate in the OFF state when the AC voltage enables a potential at thefirst input terminal 211 of the AC-to-DC converting module 21 to behigher than a potential at the second input terminal 212 of the same,and such that the second and third converting switches 216, 217 operatein the ON state and the first and fourth converting switches 215, 218operate in the OFF state when the AC voltage enables the potential atthe first input terminal 211 to be lower than the potential at thesecond input terminal 212. As a result, the AC voltage is converted intothe DC voltage.

Referring to FIGS. 3 to 6, the control module 27 controls operation ofthe first control switch 23 between an ON state and an OFF state suchthat the first control switch 23 starts to operate in the ON state at atransition time point (t₁) for a predetermined time period (T_(ON))during each predetermined cycle (T) starting from a start time point(t₀), and such that a time difference between the transition time point(t₁) and the start time point (t₀) is variable. In this embodiment, thestart time point (t₀) for each cycle (T) is a respective time point atwhich a first order derivative of the AC voltage is equal to zero(meanwhile, the source current I_(S)(t) is equal to zero, and avibration displacement of the energy source 3 reaches its extremum), andthe cycle (T) and the predetermined time period (T_(ON)) are expressedby the following equations, respectively:

$\begin{matrix}{{T = {0.5/f}},{and}} & {{equation}\mspace{14mu} 1} \\{{T_{ON} = \frac{0.5}{2\pi\sqrt{C_{P}L_{22}}}},} & {{equation}\mspace{14mu} 2}\end{matrix}$where f denotes the operating frequency of the energy source 3, C_(P)denotes a capacitance of the clamp capacitor 33 of the energy source 3,and L₂₂ denotes an inductance of the inductor 22. Therefore, anoscillation between the clamp capacitor 33 and the inductor 22 lasts foronly half of what would otherwise be the period of oscillation had thefirst control switch 23 been constantly in the ON state (i.e., if thefirst control switch 23 had been omitted), and thus a large change inthe AC voltage is expected. In this embodiment, the control module 27obtains, based on the detected AC voltage, the operating frequency ofthe energy source 3 and the time points at which the first orderderivative of the AC voltage is equal to zero. However, in otherembodiments, the control module 27 may instead detect the vibrationdisplacement of the energy source 3, and obtain, based on the detectedvibration displacement, the operating frequency of the energy source 3and the time points at which the first order derivative of the ACvoltage is equal to zero.

The control module 27 further adjusts, for each cycle (T), the timedifference between the transition time point (t₁) and the start timepoint (t₀) to change a phase difference (φ) between a center time pointof the predetermined time period (T_(ON)) and a closest time point atwhich the first order derivative of the AC voltage is equal to zero, andalso further adjusts the impedance of the impedance emulation module 26to change a voltage (V_(OUT)) across the filtering capacitor 25 in sucha way that the output power of the energy source 3 is increased.Theoretically, when the time difference between the transition timepoint (t₁) and the start time point (t₀) is adjusted to equal a targettime difference (T_(TARGET)) that makes the phase difference (φ) equal atarget phase difference (φ_(TARGET)) and when the impedance of theimpedance emulation module 26 is adjusted to equal a value that makesthe voltage (V_(OUT)) equal a target voltage (V_(TARGET)), impedances attwo sides of an interface (A-A′) are complex conjugates of each other,and thus the output power of the energy source 3 reaches its maximum.The target phase difference (φ_(TARGET)) the target time difference(T_(TARGET)) and the target voltage (V_(TARGET)) are expressed by thefollowing equations, respectively:

$\begin{matrix}{{\phi_{TARGET} = {\arctan\frac{X_{S}(\omega)}{R_{S}}}},} & {{equation}\mspace{14mu} 3} \\{{T_{TARGET} = {\frac{\phi_{TARGET}}{\omega} - {0.5T_{ON}} + {nT}}},{and}} & {{equation}\mspace{14mu} 4} \\{{V_{TARGET} = {\frac{\pi}{8}\frac{1}{\cos\;\phi_{TARGET}}V_{S}}},} & {{equation}\mspace{14mu} 5}\end{matrix}$where R_(S) and X_(S)(ω) denote a resistance and a reactance of theimpedance circuit 32 of the energy source 3, respectively, ω=2πf, n isan integer that is greater than or equal to zero and that makesT_(TARGET)≧0 and V_(S) denotes an amplitude of the source voltage. Inthis embodiment, R_(S)=R₃₂₁ and X_(S)(ω)=ωL₃₂₂−1/ωC₃₂₃ where R₃₂₁, L₃₂₂and C₃₂₃ respectively denote a resistance of the source resistor 321, aninductance of the source inductor 322 and a capacitance of the sourcecapacitor 323. As shown in FIG. 4, when the operating frequency of theenergy source 3 is smaller than a short circuit resonant frequency ofthe same (i.e., 1/√{square root over (L₃₂₂C₃₂₃)}), the phase difference(φ) is negative. As shown in FIG. 5, when the operating frequency isequal to the short circuit resonant frequency, the phase difference (φ)is equal to zero. As shown in FIG. 6, when the operating frequency isgreater than the short circuit resonant frequency, the phase difference(φ) is positive.

The control module 27 controls operation of the second control switch 24between an ON state and an OFF state such that the second control switch24 operates in the OFF state when the first control switch 23 operatesin the ON state, and such that the second control switch 24 operates inthe ON state when the first control switch 23 operates in the OFF state.

It is noted that, in other embodiments, the following modifications maybe made to the first preferred embodiment:

1. The second control switch 24 may be omitted. In this case, thefiltering capacitor 25 is coupled between the first and second outputterminals 213, 214 of the AC-to-DC converting module 21.

2. The impedance emulation module 26 may be omitted.

3. The start time point (t₀) of each cycle (T) may not be the respectivetime point at which the first order derivative of the AC voltage isequal to zero.

4. Each of the first to fourth converting switches 215-218 of theAC-to-DC converting module 21 may be replaced by a diode. In this case,the AC-to-DC converting module 21 converts the AC voltage into the DCvoltage without the control of the control module 27.

5. The control module 27 may obtain an output power of the energyharvesting device 2 based on, for example, the voltage (V_(OUT)) and theimpedance of the impedance emulation module 26, and may adjust the timedifference between the transition time point (t₁) and the start timepoint (t₀) and the impedance of the impedance emulation module 26 suchthat the output power of the energy harvesting device 2 is increased.

In view of the above, it is known from a simulation result shown in FIG.7 that, in this embodiment, since the control module 27 adjusts the timedifference between the transition time point (t₁) and the start timepoint (t₀) to make the phase difference (φ) equal the target phasedifference (φ_(TARGET) and adjusts the impedance of the impedanceemulation module 26 to make the voltage (V_(OUT)) equal the targetvoltage (V_(TARGET)) the output power of the energy source 3 and thusthe energy harvested by the energy harvesting device 2 from the energysource 3 do not change with deviation of the operating frequency of theenergy source 3 from the short circuit resonant frequency of the same.Moreover, even if the voltage (V_(OUT)) is not equal to the targetvoltage (V_(TARGET)), the energy harvested by the energy harvestingdevice 2 from the energy source 3 decreases less rapidly with thedeviation of the operating frequency from the short circuit resonantfrequency as compared to the first conventional energy harvesting device11 (see FIG. 1).

Referring to FIGS. 3 to 6, the second preferred embodiment of an energyharvesting device 2 according to this invention is a modification of thefirst preferred embodiment. Unlike the first preferred embodiment, thecontrol module 27 of the second preferred embodiment only detects the ACvoltage, and does not detect the AC current nor does it obtain theoutput power of the energy source 3. Moreover, the control module 27obtains the target time difference (T_(TARGET)) of Equation 4 based onthe resistance and the reactance of the impedance circuit 32 of theenergy source 3, on the operating frequency of the energy source 3 andon the predetermined time period (T_(ON)), and controls the timedifference between the transition time point (t₁) and the start timepoint (t₀) to be equal to the target time difference (T_(TARGET)). Thecontrol module 27 further obtains the target voltage (V_(TARGET)) ofEquation 5 based on the resistance and the reactance of the impedancecircuit 323 of the energy source 3 and on the amplitude of the sourcevoltage, and controls the impedance of the impedance emulation module 26based on the target voltage (V_(TARGET)) such that the voltage (V_(OUT))is equal to the target voltage (V_(TARGET)).

It is noted that the second preferred embodiment is suitable under thecircumstance where the resistance and the reactance of the impedancecircuit 323 of the energy source 3 and the amplitude of the sourcevoltage can be known precisely. On the contrary, the first preferredembodiment is suitable for various situations. Moreover, in acircumstance where the resistance and the reactance of the impedancecircuit 323 of the energy source 3 and the amplitude of the sourcevoltage can be known approximately, the first and second preferredembodiments can be combined. In such case, the approximate target timedifference (T_(TARGET)) is obtained based on Equation 4 and serves as aninitial value of the time difference between the transition time point(t₁) and the start time point (t₀), the approximate target voltage(V_(TARGET)) is obtained based on Equation 5, an initial value of theimpedance of the impedance emulation module 26 is obtained based on theapproximate target voltage (V_(TARGET)) and the time difference betweenthe transition time point (t₁) and the start time point (t₀) and theimpedance of the impedance emulation module 26 are then adjusted fromthese initial values, thereby reducing the overall adjustment time.

Referring to FIG. 8, the third preferred embodiment of an energyharvesting device 2 according to this invention is a modification of thefirst preferred embodiment. Unlike the first preferred embodiment, theenergy harvesting device 2 of the third preferred embodiment does notinclude the AC-to-DC converting module 21 (see FIG. 3) and the filteringcapacitor 25 (see FIG. 3). Therefore, the energy harvesting device 2 ofthe third preferred embodiment does not convert the AC voltage into a DCvoltage. Moreover, the series connection of the second control switch 24and the impedance emulation module 26 is coupled to the seriesconnection of the inductor 22 and the first control switch 23 inparallel.

It is noted that, in other embodiments, the second control switch 24 maybe omitted. In this case, the impedance emulation module 26 is coupledto the series connection of the inductor 22 and the first control switch23 in parallel.

Referring to FIG. 9, the fourth preferred embodiment of an energyharvesting device 4 according to this invention harvests energy from theaforesaid energy source 3. The energy harvesting device 4 usesseries-SSHI techniques, and includes an AC-to-DC converting module 41,first and second inductors 42, 46, first and second control switches 43,47, a filtering capacitor 44, a diode 45, an impedance emulation module48 and a control module 49.

The AC-to-DC converting module 41 includes first and second inputterminals 411, 412 adapted to be coupled to the energy source 3 forreceiving the AC voltage therefrom, first and second output terminals413, 414 for outputting a DC voltage, and first to fourth convertingdiodes 415-418. The first converting diode 415 has an anode coupled tothe first input terminal 411, and a cathode coupled to the first outputterminal 413. The second converting diode 416 has an anode coupled tothe second input terminal 412, and a cathode coupled to the first outputterminal 413. The third converting diode 417 has an anode coupled to thesecond output terminal 414, and a cathode coupled to the first inputterminal 411. The fourth converting diode 418 has an anode coupled tothe second output terminal 414, and a cathode coupled to the secondinput terminal 412. The first to fourth converting diodes 415-418cooperatively convert the AC voltage into the DC voltage.

The impedance emulation module 48 is coupled to the filtering capacitor44 in parallel, and has a variable impedance. The first inductor 42, thefirst control switch 43 and the parallel connection of the filteringcapacitor 44 and the impedance emulation module 48 are coupled in seriesbetween the first and second output terminals 413, 414 of the AC-to-DCconverting module 41 (i.e., the series connection of the first inductor42 and the first control switch 43 is coupled to the energy source 3 inseries via the first converting diode 415). The diode 45 has an anodecoupled to the second output terminal 414 of the AC-to-DC convertingmodule 41, and a cathode coupled to the first output terminal 413 of theAC-to-DC converting module 41. The diode 45 is capable of releasingenergy stored in the first inductor 42. The second inductor 46 and thesecond control switch 47 are coupled in series between the first andsecond input terminals 411, 412 of the AC-to-DC converting module 41.The impedance emulation module 48 may use techniques disclosed in“Resistor Emulation Approach to Low-Power RF Energy Harvesting” by T.Paing et al., IEEE Transactions on Power Electronics, vol. 23, no. 3,pp. 1494, 2008, and details thereof are omitted herein for the sake ofbrevity.

The control module 49 is coupled to the first and second controlswitches 43, 47 and the impedance emulation module 48, is adapted to becoupled to the energy source 3 for detecting the AC voltage and the ACcurrent therefrom, and obtains an output power of the energy source 3based on the detected AC voltage and the detected AC current.

Referring further to FIGS. 10 to 12, the control module 49 controlsoperation of the first control switch 43 between an ON state and an OFFstate such that the first control switch 43 starts to operate in the ONstate for a first predetermined time period (T_(ON1)) from a transitiontime point (t₁) during each cycle (T) starting from a start time point(t₀), and such that a time difference between the transition time point(t₁) and the start time point (t₀) is variable. The control module 49controls operation of the second control switch 47 between an ON stateand an OFF state such that the second control switch 47 operates in theON state for a second predetermined time period (T_(ON2)) immediatelyafter each transition of the first control switch 43 from the ON stateto the OFF state. In this embodiment, the start time point (t₀) of eachcycle (T) is a respective time point at which a first order derivativeof the AC voltage is equal to zero (meanwhile, the source currentI_(S)(t) is also equal to zero, and a vibration displacement of theenergy source 3 reaches its extremum), and the cycle (T), the firstpredetermined time period (T_(ON1)) and the second predetermined timeperiod (T_(ON2)) are expressed by the following equations, respectively:

$\begin{matrix}{{T = {0.5/f}},} & {{equation}\mspace{14mu} 6} \\{{T_{{ON}\; 1} = \frac{0.25}{2\pi\sqrt{C_{P}L_{42}}}},{and}} & {{equation}\mspace{14mu} 7} \\{{T_{{ON}\; 2} = \frac{0.5}{2\pi\sqrt{C_{P}L_{46}}}},} & {{equation}\mspace{14mu} 8}\end{matrix}$where f denotes the operating frequency of the energy source 3, C_(P)denotes a capacitance of the clamp capacitor 33 of the energy source 3,L₄₂ denotes an inductance of the first inductor 42, and L₄₆ denotes aninductance of the second inductor 46. Thus, a large change in the ACvoltage during a total ON time period (T_(ON)) including the first andsecond predetermined time periods (T_(ON1), T_(ON2)) is expected. Inthis embodiment, the control module 49 obtains, based on the detected ACvoltage, the operating frequency of the energy source 3 and the timepoints at which the first order derivative of the AC voltage is equal tozero. However, in other embodiments, the control module 49 may detectthe vibration displacement of the energy source 3, and obtain, based onthe detected vibration displacement, the operating frequency of theenergy source 3 and the time points at which the first order derivativeof the AC voltage is equal to zero.

It should be noted herein that while the second control switch 47 isshown to be switched to the ON state immediately after the first controlswitch 43 is switched to the OFF state in this embodiment, such“immediate” transition of the second control switch 47 into the ON stateupon transition of the first control switch 43 into the OFF state is nota necessary limitation in other embodiments of the present invention.

The control module 49 further adjusts, for each cycle (T), the timedifference between the transition time point (t₁) and the start timepoint (t₀) to change a phase difference (φ) between a center time pointof the total ON time period (T_(ON)) and a closest time point at whichthe first order derivative of the AC voltage is equal to zero, and alsofurther adjusts the impedance of the impedance emulation module 48 tochange a voltage (V_(OUT)) across the filtering capacitor 44 in suchaway that the output power of the energy source 3 is increased.Theoretically, when the time difference between the transition timepoint (t₁) and the start time point (t₀) is adjusted to equal a targettime difference (T_(TARGET)) that makes the phase difference (φ) equal atarget phase difference (φ_(TARGET)), and when the impedance of theimpedance emulation module 48 is adjusted to equal a value that makesthe voltage (V_(OUT)) equal a target voltage (V_(TARGET)), impedances attwo sides of an interface (A-A′) are complex conjugates of each other,and thus the output power of the energy source 3 reaches its maximum.The target phase difference (φ_(TARGET)) the target time difference(T_(TARGET)) and the target voltage (V_(TARGET)) are expressed by thefollowing equations, respectively:

$\begin{matrix}{{\phi_{TARGET} = {\arctan\frac{{X_{S}(\omega)} - {{1/\omega}\; C_{P}}}{R_{S}}}},} & {{equation}\mspace{14mu} 9} \\{{T_{TARGET} = {\frac{\phi_{TARGET}}{\omega} - {0.5T_{ON}} + {nT}}},{and}} & {{equation}\mspace{14mu} 10} \\{{V_{TARGET} = {\left\lbrack {{\frac{\pi}{8}\frac{1}{\cos\;\phi_{TARGET}}} - \frac{\cos\;\phi_{TARGET}}{2\omega\; R_{S}C_{P}}} \right\rbrack V_{S}}},} & {{equation}\mspace{14mu} 11}\end{matrix}$where R_(S) and X_(S)(ω) respectively denote a resistance and areactance of the impedance circuit 32 of the energy source 3, −1/ωC_(P)denotes a reactance of the clamp capacitor 33 of the energy source 3,ω=2πf, n is an integer that is greater than or equal to zero and thatmakes T_(TARGET)≧0 and V_(S) denotes an amplitude of the source voltage.In this embodiment, R_(S)=R₃₂₁, X_(S)(ω)=ωL₃₂₂−1/ωC₃₂₃, where R₃₂₁, L₃₂₂and C₃₂₃ respectively denote a resistance of the source resistor 321, aninductance of the source inductor 322 and a capacitance of the sourcecapacitor 323 of the impedance circuit 32 of the energy source 3. Asshown in FIG. 10, when the operating frequency of the energy source 3 issmaller than an open circuit resonant frequency of the same (i.e.,1/√{square root over (L₃₂₂C₃₂₃C_(P)/(C₃₂₃+C_(P)))}) the phase difference(φ) is negative. As shown in FIG. 11, when the operating frequency isequal to the open circuit resonant frequency, the phase difference (φ)is equal to zero. As shown in FIG. 12, when the operating frequency isgreater than the open circuit resonant frequency, the phase difference(φ) is positive.

It is noted that, in other embodiments, the following modifications maybe made to the fourth preferred embodiment:

1. The diode 45 may be omitted.

2. The second inductor 46 and the second control switch 47 may beomitted. In this case, the total ON time period (T_(ON)) only includesthe first predetermined time period (T_(ON1)) of the first controlswitch 43.

3. The impedance emulation module 48 may be omitted.

4. The start time point (t₀) for each cycle (T) may not be therespective time point at which the first order derivative of the ACvoltage is equal to zero.

5. Each of the first to fourth converting diodes 415-418 of the AC-to-DCconverting module 41 may be replaced by a switch. In this case, thecontrol module 49 is further coupled to the AC-to-DC converting module41, and controls, based on the detected AC voltage, operation of each ofthe switches between an ON state and an OFF state such that the ACvoltage is converted into the DC voltage.

6. The control module 49 may obtain an output power of the energyharvesting device 4 based on, for example, the voltage (V_(OUT)) and theimpedance of the impedance emulation module 48, and adjust the timedifference between the transition time point (t₁) and the start timepoint (t₀) and the impedance of the impedance emulation module 48 suchthat the output power of the energy harvesting device 4 is increased.

7. The diode 45 may be replaced by a switch. In this case, the controlmodule 49 is further coupled to the switch, and controls operation ofthe switch between an ON state and an OFF state such that the switchoperates in the ON state for a predetermined time period immediatelyafter each transition of the first control switch 43 from the ON stateto the OFF state.

8. As shown in FIG. 13, when the first inductor 42 is coupled betweenthe first control switch 43 and the filtering capacitor 44, the cathodeof the diode 45 may be coupled to a common node between the firstinductor 42 and the first control switch 43, instead of the first outputterminal 413 of the AC-to-DC converting module 41.

9. As shown in FIG. 14, the series connection of the first inductor 42and the first control switch 43 may be coupled between the energy source3 and the first input terminal 411 of the AC-to-DC converting module 41,with the first control switch 43 coupled to the energy source 3 and thefirst inductor 42 coupled to the first input terminal 411 of theAC-to-DC converting module 41. The series connection of the secondinductor 46 and the second control switch 47 may be coupled to theseries connection of the first inductor 42 and the first control switch43 in series between the first and second input terminals 411, 412 ofthe AC-to-DC converting module 41 as shown; alternatively, the secondinductor 46 and the second control switch 47 may be omitted. The diode45 (see FIG. 9) may be replaced by a switch 45′ that is coupled betweena common node between the first inductor 42 and the first control switch43 and the second input terminal 412 of the AC-to-DC converting module41 and that is controlled by the control module 49 as shown;alternatively, the diode 45 (see FIG. 9) may be omitted. In this case,the filtering capacitor 44 is coupled between the first and secondoutput terminals 413, 414 of the AC-to-DC converting module 41.

In view of the above, in this embodiment, since the control module 49adjusts the time difference between the transition time point (t₁) andthe start time point (t₀) to make the phase difference (φ) equal thetarget phase difference (φ_(TARGET)) and adjusts the impedance of theimpedance emulation module 48 to make the voltage (V_(OUT)) equal thetarget voltage (V_(TARGET)), the output power of the energy source 3 andthus the energy harvested by the energy harvesting device 4 from theenergy source 3 do not change with deviation of the operating frequencyof the energy source 3 from the open circuit resonant frequency of thesame. Moreover, even if the voltage (V_(OUT)) is not equal to the targetvoltage (V_(TARGET)) the energy harvested by the energy harvestingdevice 4 from the energy source 3 decreases less rapidly with thedeviation of the operating frequency from the open circuit resonantfrequency as compared to the second conventional energy harvestingdevice 11′ (see FIG. 2).

Referring to FIGS. 9 to 12, the fifth preferred embodiment of an energyharvesting device 4 according to this invention is a modification of thefourth preferred embodiment. Unlike the fourth preferred embodiment, thecontrol module 49 of the fifth preferred embodiment only detects the ACvoltage, and does not detect the AC current nor does it obtain theoutput power of the energy source 3. Moreover, the control module 49obtains the target time difference (T_(TARGET)) of Equation 10 based onthe resistance and the reactance of the impedance circuit 32 of theenergy source 3, on the reactance of the clamp capacitor 33 of theenergy source 3, on the operating frequency of the energy source 3 andon the total ON time period (T_(ON)), and controls the time differencebetween the transition time point (t₁) and the start time point (t₀) toequal the target time difference (T_(TARGET)). The control module 27further obtains the target voltage (V_(TARGET)) of Equation 11 based onthe resistance and the reactance of the impedance circuit 323 of theenergy source 3, on the reactance of the clamp capacitor 33 of theenergy source 3 and on the amplitude of the source voltage, and controlsthe impedance of the impedance emulation module 48 based on the targetvoltage (V_(TARGET)) such that the voltage (V_(OUT)) is equal to thetarget voltage (V_(TARGET)).

It is noted that the fifth preferred embodiment is suitable under thecircumstance where the resistance and the reactance of the impedancecircuit 323 of the energy source 3, the reactance of the clamp capacitor33 of the energy source 3 and the amplitude of the source voltage can beknown precisely. On the contrary, the fourth preferred embodiment issuitable for various situations. Moreover, in a circumstance where theresistance and the reactance of the impedance circuit 323 of the energysource 3, the reactance of the clamp capacitor 33 of the energy source 3and the amplitude of the source voltage can be known approximately, thefourth and fifth preferred embodiments can be combined. In such case,the approximate target time difference (T_(TARGET)) is obtained based onEquation 10 and serves as an initial value of the time differencebetween the transition time point (t₁) and the start time point (t₀),the approximate target voltage (V_(TARGET)) is obtained based onEquation 11, an initial value of the impedance of the impedanceemulation module 48 is obtained based on the approximate target voltage(V_(TARGET)) and the time difference between the transition time point(t₁) and the start time point (t₀) and the impedance of the impedanceemulation module 48 are then adjusted from these initial values, therebyreducing the overall adjustment time.

Referring to FIG. 15, the sixth preferred embodiment of an energyharvesting device 4 according to this invention is a modification of thefourth preferred embodiment. Unlike the fourth preferred embodiment, theenergy harvesting device 4 of the sixth preferred embodiment does notinclude the AC-to-DC converting module 41 (see FIG. 9), the filteringcapacitor 44 (see FIG. 9), the diode 45 (see FIG. 9), the secondinductor 46 (see FIG. 9) and the second control switch 47 (see FIG. 9).Therefore, the energy harvesting device 4 of the sixth preferredembodiment does not convert the AC voltage into a DC voltage. Moreover,the series connection of the first inductor 42 and the first controlswitch 43 is coupled between the energy source 3 and the impedanceemulation module 48 (i.e., the impedance emulation module 48 is coupledin series to the series connection of the first inductor 42 and thefirst control switch 43).

While this invention has been described in connection with what areconsidered the most practical and preferred embodiments, it isunderstood that this invention is not limited to the disclosedembodiments but is intended to cover various arrangements includedwithin the spirit and scope of the broadest interpretation andequivalent arrangements.

What is claimed is:
 1. An energy harvesting device for harvesting energyfrom an energy source that outputs an alternating current (AC) voltageand an AC current, said energy harvesting device comprising: an AC-to-DC(direct current) converting module having two input terminals that areadapted to be coupled to the energy source for receiving the AC voltagetherefrom, and two output terminals, said AC-to-DC converting moduleconverting the AC voltage into a DC voltage, and outputting the DCvoltage at said output terminals; an inductor and a control switchcoupled in series between said input terminals of said AC-to-DCconverting module; a filtering capacitor coupled between said outputterminals of said AC-to-DC converting module; and a control modulecoupled to said control switch, and controlling operation of saidcontrol switch between an ON state and an OFF state such that saidcontrol switch starts to operate in the ON state for a predeterminedtime period from a transition time point during each predetermined cyclestarting from a start time point, and such that a time differencebetween the transition time point and the start time point is variable;said control module being adapted to be coupled to the energy source fordetecting the AC voltage and the AC current therefrom, said controlmodule obtaining an output power of the energy source based on thedetected AC voltage and the detected AC current, and adjusting the timedifference between the transition time point and the start time pointsuch that the output power of the energy source is increased.
 2. Theenergy harvesting device of claim 1, wherein the start time point is arespective time point at which a first order derivative of the ACvoltage is equal to zero.
 3. The energy harvesting device of claim 1,further comprising an impedance emulation module that is coupled to saidfiltering capacitor in parallel, and that has a variable impedance;wherein said control module is further coupled to said impedanceemulation module, and adjusts the impedance of said impedance emulationmodule to change a voltage across said filtering capacitor such that theoutput power of the energy source is increased.
 4. The energy harvestingdevice of claim 1, the energy source operating at an operatingfrequency, and equivalently including an AC supply and an impedancecircuit, the AC supply supplying a source voltage to the impedancecircuit such that the impedance circuit outputs the AC voltage; whereinsaid control module further obtains a target time difference based on aresistance and a reactance of the impedance circuit of the energysource, on the operating frequency and on the predetermined time period,and controls the time difference between the transition time point andthe start time point to be equal to the target time difference.
 5. Theenergy harvesting device of claim 4, wherein the target time differenceis obtained using the following equations:${T_{TARGET} = {\frac{\phi_{TARGET}}{\omega} - {0.5T_{ON}} + {nT}}},{\phi_{TARGET} = {\arctan\frac{X_{S}(\omega)}{R_{S}}}},{\omega = {2\pi\; f}},{and}$T = 0.5f, where T_(TARGET) denotes the target time difference, R_(S) andX_(S)(ω) respectively denote the resistance and the reactance of theimpedance circuit of the energy source, f denotes the operatingfrequency, T_(ON) denotes the predetermined time period, and n is aninteger greater than or equal to zero.
 6. The energy harvesting deviceof claim 1, the energy source operating at an operating frequency, andequivalently including an AC supply and an impedance circuit, the ACsupply supplying a source voltage to the impedance circuit such that theimpedance circuit outputs the AC voltage, said energy harvesting devicefurther comprising an impedance emulation module that is coupled to saidfiltering capacitor in parallel, and that has a variable impedance;wherein said control module is further coupled to said impedanceemulation module, obtains a target voltage based on a resistance and areactance of the impedance circuit of the energy source and on anamplitude of the source voltage, and controls the impedance of saidimpedance emulation module based on the target voltage such that avoltage across said filtering capacitor is equal to the target voltage.7. The energy harvesting device of claim 6, wherein the target voltageis obtained using the following equations:${V_{TARGET} = {\frac{\pi}{8}\frac{1}{\cos\;\phi_{TARGET}}V_{S}}},{and}$${\phi_{TARGET} = {\arctan\;\frac{X_{S}(\omega)}{R_{S}}}},$ whereV_(TARGET) denotes the target voltage, R_(S) and X_(S)(ω) respectivelydenote the resistance and the reactance of the impedance circuit of theenergy source, and V_(S) denotes the amplitude of the source voltage. 8.An energy harvesting device for harvesting energy from an energy sourcethat outputs an alternating current (AC) voltage and an AC current, saidenergy harvesting device comprising: an AC-to-DC (direct current)converting module having two input terminals that are adapted to becoupled to the energy source for receiving the AC voltage therefrom, andtwo output terminals, said AC-to-DC converting module converting the ACvoltage into a DC voltage, and outputting the DC voltage at said outputterminals; a first inductor, a first control switch and a filteringcapacitor coupled in series between said output terminals of saidAC-to-DC converting module; and a control module coupled to said firstcontrol switch, and controlling operation of said first control switchbetween an ON state and an OFF state such that said first control switchstarts to operate in the ON state for a first predetermined time periodfrom a transition time point during each predetermined cycle startingfrom a start time point, and such that a time difference between thetransition time point and the start time point is variable; said controlmodule being adapted to be coupled to the energy source for detectingthe AC voltage and the AC current therefrom, said control moduleobtaining an output power of the energy source based on the detected ACvoltage and the detected AC current, and adjusting the time differencebetween the transition time point and the start time point such that theoutput power of the energy source is increased.
 9. The energy harvestingdevice of claim 8, wherein the start time point is a respective timepoint at which a first order derivative of the AC voltage is equal tozero.
 10. The energy harvesting device of claim 8, further comprising animpedance emulation module that is coupled to said filtering capacitorin parallel, and that has a variable impedance; wherein said controlmodule is further coupled to said impedance emulation module, andadjusts the impedance of said impedance emulation module to change avoltage across said filtering capacitor such that the output power ofthe energy source is increased.
 11. The energy harvesting device ofclaim 8, the energy source operating at an operating frequency, andequivalently including an AC supply, an impedance circuit and a clampcapacitor, the AC supply supplying a source voltage to the impedancecircuit, the impedance circuit outputting the AC voltage across theclamp capacitor; wherein said control module further obtains a targettime difference based on a resistance and a reactance of the impedancecircuit of the energy source, on a reactance of the clamp capacitor ofthe energy source, on the operating frequency and on a total ON timeperiod that includes the first predetermined time period, and controlsthe time difference between the transition time point and the start timepoint to be equal to the target time difference.
 12. The energyharvesting device of claim 11, wherein the target time difference isobtained using the following equations:${T_{TARGET} = {\frac{\phi_{TARGET}}{\omega} - {0.5T_{ON}} + {nT}}},{\phi_{TARGET} = {\arctan\frac{{X_{S}(\omega)} - {{1/\omega}\; C_{P}}}{R_{S}}}},{\omega = {2\pi\; f}},{and}$T = 0.5f, where T_(TARGET) denotes the target time difference, R_(S) andX_(S)(ω) respectively denote the resistance and the reactance of theimpedance circuit of the energy source, —1/ωC_(P) denotes the reactanceof the clamp capacitor of the energy source, f denotes the operatingfrequency, T_(ON) denotes the total ON time period, and n is an integergreater than or equal to zero.
 13. The energy harvesting device of claim8, the energy source operating at an operating frequency, andequivalently including an AC supply, an impedance circuit and a clampcapacitor, the AC supply supplying a source voltage to the impedancecircuit such that the impedance circuit outputs the AC voltage acrossthe clamp capacitor, said energy harvesting device further comprising animpedance emulation module that is coupled to said filtering capacitorin parallel, and that has a variable impedance; wherein said controlmodule is further coupled to said impedance emulation module, obtains atarget voltage based on a resistance and a reactance of the impedancecircuit of the energy source, on a reactance of the clamp capacitor ofthe energy source and on an amplitude of the source voltage, andcontrols the impedance of said impedance emulation module based on thetarget voltage such that a voltage across said filtering capacitor isequal to the target voltage.
 14. The energy harvesting device of claim13, wherein the target voltage is obtained using the followingequations:${V_{TARGET} = {\left\lbrack {{\frac{\pi}{8}\frac{1}{\cos\;\phi_{TARGET}}} - \frac{\cos\;\phi_{TARGET}}{2\omega\; R_{S}C_{P}}} \right\rbrack V_{S}}},{and}$${\phi_{TARGET} = {\arctan\frac{{X_{S}(\omega)} - {{1/\omega}\; C_{P}}}{R_{S}}}},$where V_(TARGET) denotes the target voltage, R_(S) and X_(S)(ω)respectively denote the resistance and the reactance of the impedancecircuit of the energy source, −1/ωC_(P) denotes the reactance of theclamp capacitor of the energy source, and V_(S) denotes the amplitude ofthe source voltage.
 15. The energy harvesting device of claim 8, furthercomprising a second inductor and a second control switch coupled inseries between said input terminals of said AC-to-DC converting module;wherein said control module is further coupled to said second controlswitch, and controls operation of said second control switch between anON state and an OFF state such that said second control switch operatesin the ON state for a second predetermined time period after eachtransition of said first control switch from the ON state to the OFFstate.